CHEM 101 Lecture 4: Bohr Model and Quantum TheoryPremium
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CHEM 101 - Lecture 4 - Bohr Model and Quantum Theory
Note: Read sections 7.4, 8.1.
Practise problem set 2, questions 1 - 4
Bohr Model for Hydrogen Atom:
● Hydrogen atom is the simplest atom
● Classical mechanics predicts that the electron would collapse into the positive nucleus,
contrary to observations.
● Bohr’s model blends classical and quantum theories.
○ There is one (and only one) electron moving in a circular orbit around the
○ The electron has only a fixed set of allowed orbits.
○ The electron can only have a certain finite energies (also known as quantums)
○ En= -Rh (z2/ h2)
■ R = Rydberg’s constant = 2.178 x 10-18 J
■ Z = Nuclear charge = number of protons = atomic number
■ n = integer value, beginning at 1, going to infinity. n cannot equal 0
■ So for hydrogen, Z = 1
■ For He+ Z = 2, Li 2+ Z= 3
■ radius of an orbit = a x n2 where a=0.053 nm (This explains how each
subsequent orbit gets further and further away from the nucleus)
■ Ex. for n=1: E1 = -Rh x Z2 < 0 (This will be a negative value)
■ Ex. for n = infinity: E = -Rh x( Z2/
2) = 0
■ For absorption: Change in energy is a positive value
■ For emission: Change in energy is a negative value
■ Change in energy and wavelength is inversely proportional
Atomic Spectra: Hydrogen and Hydrogen like Atoms
● H atom: 1 electron, one proton, z = 1
● Bohr model: 1 electron orbits around a nucleus
● H -like atoms are really ions
○ Ex. He has 2 electrons and 2 protons, which does not conform to the Bohr
○ However, He+ has 1 electron, 2 protons, Z = 2. This follows Bohr’s model.
○ Li 2+ has 1 electron, 3 protons, z =3. This follows Bohr’s model.
○ Li and Li+ will not work because there is more than one electron orbiting around
● General formula for H - like atom: E+ (Z -1)
○ Only certain energy levels are allowed.
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